In the past 40 years, countries all over the world have spared no effort in developing new materials for light-emitting devices (LED) and improving their internal quantum efficiency. However, the external quantum efficiency of LEDs is still far behind the internal quantum efficiency. The internal quantum efficiency of LEDs with double heterojunction (DH) structure can be up to 99%, while the external quantum efficiency is only several percent, which is a result of the following reasons: (1) The electric current distribution of the p-type cladding layer, the p-type window layer and the p-type contact layer on the light-emitting layer of the LED results in that most photons are reflected back by the p-type ohmic contact electrode and finally absorbed by the substrate, which reduces the probability of photons emitted out of the LED; (2) It is difficult for photons to be emitted to the air with low refractivity index (n=1) from semiconductor materials with high refractivity index, and most photons are absorbed by the substrate.
Commercial high-luminance LEDs, such as red, yellow, green and blue LEDs, all presently use either a aluminum gallium indium phosphide/gallium arsenide substrate or a aluminum indium gallium nitride/sapphire single crystal substrate, and use Metal-Organic Chemical Vapor Deposition (MOCVD) technology to grow the epitaxial film of the LED.
FIG. 1 is a cross-sectional diagram of an LED 10 according to the prior art. As shown in FIG. 1, the LED 10 comprises a gallium arsenide (GaAs) substrate 12, a bottom cladding layer 14 composed of n-type aluminum gallium indium phosphide (n-AlGaInP), a light-emitting layer 16 composed of undoped aluminum gallium indium phosphide, and an upper cladding layer 18 composed of p-type aluminum gallium indium phosphide (p-AlGaInP). In addition, the LED 10 also comprises a p-type ohmic contact electrode 22 positioned on the surface of the upper cladding layer 18, and an n-type ohmic contact electrode 20 positioned on the bottom surface of the gallium arsenide substrate 12. The p-type ohmic contact electrode 22 is a metallic film with a diameter about 150 micrometers for wire bonding. The current applied to the LED 10 flows from the p-type ohmic contact electrode 22 via the upper cladding layer 18 to the light-emitting layer 16 where photons are generated.
Since the upper cladding layer 18 of p-type aluminum gallium indium phosphide has a higher resistance and small thickness, it is difficult for the current to be spread laterally and uniformly, which results in that most of the current is concentrated right below the p-type ohmic contact electrode 22. However, photons generated at the light-emitting layer 16 right below the p-type ohmic contact electrode 22 are screened by the p-type ohmic contact electrode 22 and reflected back when the photons try to emit out of the LED 10, and most photons will be absorbed finally by the gallium arsenide substrate 12 with a smaller band gap. As a result, the external quantum efficiency of the LED 10 is restricted.
The LED 10 uses the upper cladding 18 composed of p-type aluminum gallium indium phosphide to spread the current directly, which results in three problems: (1) The carrier mobility of the p-type aluminum gallium indium phosphide is quite low, only about 10 (cm2/V-s); (2) It is difficult to perform an ionic implanting process on the p-type aluminum gallium indium phosphide such that the maximum carrier concentration is only about 1018/cm3; (3) When the thickness of the p-type aluminum gallium indium phosphide is decreased to a range between 2 and 5 micrometers, the quality of the aluminum gallium indium phosphide is inferior.
FIG. 2 is a cross-sectional diagram of an LED 30 according to the prior art. Compared with the LED 20 in FIG. 1, the LED 30 further comprises a p-type window layer 32 on the upper cladding layer 18. The p-type window layer 32 with a thickness between 2 and 50 micrometers is epitaxially grown on the upper cladding layer 18 to overcome the above-mentioned problems. Currently, semiconductor materials widely used to form the window layer include indium aluminum phosphide, aluminum gallium arsenide, gallium nitride, gallium phosphide, etc. The p-type window layer 32 not only possesses a lower resistance, but is also transparent to photons emitted from the light-emitting layer 16, namely photons will not be absorbed. Researchers have successfully developed an aluminum gallium indium phosphide high-luminance red LED using the p-type window layer 32. Toshiba Corporation in Japan used the p-type aluminum gallium arsenide to prepare the window layer and the current spreading layer in 1991, which increases the external quantum efficiency of the LED by about 40 times. After that, HP Corporation in America used the p-type gallium arsenide with a thickness between 2 and 15 micrometers to prepare the window layer, which effectively promotes the external quantum efficiency of the LED.
FIG. 3 is a cross-sectional diagram of an LED 40 according to the prior art. Compared with the LED 30 in FIG. 2, the LED 40 uses a window layer 42 on the upper cladding layer 18 to spread the current laterally, wherein the window layer 42 uses a three-layer epitaxial layer structure, namely gallium phosphide/gallium arsenide/gallium phosphide. The United Photoelectric Corporation in Taiwan has used this film structure to successfully develop an aluminum gallium indium phosphide high-luminance red LED.
FIG. 4 is a cross-sectional diagram of an LED 50 according to the prior art, which uses a transparent electrode 52 to spread current laterally. The transparent electrode 52 composed of indium tin oxide (ITO) is grown on the upper cladding layer 18 composed of p-type aluminum gallium indium phosphide, thus the electric current of the LED 50 is spread to the upper cladding layer 18 laterally via the transparent electrode 52. The Industrial Technology Research Institute of Taiwan, ROC, used this film structure to develop the aluminum gallium indium phosphide high-luminance red LED successfully. However, since it is difficult to form a good ohmic contact property between the ITO and the aluminum gallium indium phosphide, a graded layer 54 composed of p-type gallium arsenide is required to be added between the ITO and the aluminum gallium indium phosphide.
FIG. 5 is a cross-sectional diagram of an LED 60 according to the prior art. The LED 60 uses a current blocking structure to force the current to spread laterally, and comprises a p-type window layer 62 on the upper cladding layer 18 and an n-type epitaxial layer 64 between the upper cladding layer 18 and the p-type window layer 62. The n-type epitaxial layer 64 is right below the p-type ohmic contact electrode 22, and forms a p-n junction with the upper cladding 18 for blocking the flow of the current therethrough. The current blocking structure of the LED 60 can effectively spread the current laterally to the region around the p-type ohmic contact electrode 22, rather than concentrate around the central region. As a result, the probability of photons generated at the light-emitting layer 16 and screened by the p-type ohmic contact electrode 22 is decreased, and the external quantum efficiency of the LED 60 is increased. Toshiba Corporation in Japan prepared an LED with the window layer of p-type aluminum gallium arsenide and the n-type island current blocking structure.
If the current blocking structure with the p-n junction is used, two steps are needed during the MOCVD epitaxial growth. Firstly, an n-type epitaxial layer 64 with a width of tens of nanometers has to be grown on the upper cladding layer 18, and the epitaxial wafer is then moved out from the reaction chamber to form the n-type epitaxial layer 64 into an island shape through a photolithographic and an etching process. The epitaxial wafer is moved back into the MOCVD reaction chamber to grow the p-type window layer 62. In addition, although the current blocking structure of the LED 60 is positioned above the upper cladding layer 18, the current from the p-type ohmic contact electrode 22 may flow to the light-emitting layer 16 right below the n-type epitaxial layer 64, where the generated photon still can not emit out of the LED 60 due to the screen of the p-type ohmic contact electrode 22.
Presently, there are many available technologies to solve the issue of electric current spreading for the LED. For instance, Sugawara et al. use the selected area diffusion method to prepare the current blocking structure (SEE U.S. Pat. No. 5,153,889). B. J. Lee et al. formed a circular hole with a depth to the upper cladding composed of p-type aluminum gallium indium phosphide in the transparent electrode composed of ITO or ZnO using photolithographic and etching technologies, and a metallic film is deposited in the hole to form an Schokky Barrier. The metallic film is heated to form a natural oxide at the interface between the metallic membrane and the p-type aluminum gallium indium phosphide, wherein the Schokky Barrier and the natural oxide form a current blocking structure (SEE to U.S. Pat. No. 5,717,226). In addition, U.S. Pat. No. 5,949,093, U.S. Pat. No. 6,420,732 B1, EP 1,225,670 A1, U.S. 2001/0050530 A1, U.S. 2003/0039288 A1 and U.S. Pat. No. 6,522,676 B1 also discloses different designs of current blocking structure.